1. Field of the Invention
The present invention relates to micro machining technology and, particularly, to a micro machining apparatus for machining a micro trench, which has a high aspect ratio and serves as, for example, a shaft hole of a micro-motor and a pressure guide hole of a pressure sensor.
2. Description of the Related Art
FIG. 6(a) shows an example of a micro machining apparatus employing an electric discharge machining technique, for machining micro trenches and holes.
In the figure, a workpiece 110 is held by a holder electrode 130 and immersed in oil 100. A machining electrode 121 is made of high-rigid material such as tungsten, which is hardly broken by internal defects even when shaped into a thin wire. A feed mechanism 120 rotates and feeds the machining electrode 121 toward the workpiece 110.
The electric discharge machining technique makes a very small hole as an accumulation of craters each formed by a shot of electric discharge. In FIG. 6(a), an RC electric discharge circuit 131 provides shots of electric discharge. Electric discharge energy E(J) of each shot is expressed as follows: EQU E=(C.multidot.V.sup.2)/2 (1)
where C is capacitance and V is a voltage applied to the electrodes.
Each shot of the electric discharge energy E bores the workpiece 110 while the machining electrode 121 is being fed toward the workpiece 110, thereby forming a very small hole 111 in the workpiece 110.
Since the apparatus employing the electric discharge machining technique forms a fine hole as an accumulation of craters each formed by a shot of electric discharge, the finished surface of the hole is rough. To provide a smoother finished surface, the discharge energy E of the equation (1) must be reduced and, to achieve this, the apparatus must have as small a floating capacitance as possible.
In addition, the material of a workpiece to be processed by the electric discharge machining apparatus must be electrically conductive.
As the depth (aspect ratio) of the hole 111 bored by the apparatus deepens, the machining electrode 121 goes deep inside the workpiece 110. As a result, electric discharge occurs not only at the leading end of the machining electrode 121 but also over the body thereof, to cause a problem that a diameter L' of the hole 111 at the surface of the workpiece 110 differs from a diameter L at the bottom of the hole 111, as shown in FIG. 6(b). The electric discharge machining technique, therefore, hardly provides a hole having a uniform diameter.
Another technique of providing a high-aspect structure is an LIGA method, i.e., a lithographic electroforming method. This method thickly applies PMMA resist over a substrate, photolithographically processes the resist with X rays to form a mold, and electrically casts metal into the mold, thereby providing a high-aspect structure.
This LIGA method, however, requires SOR (Synchrotron Orbital Radiation), which is expensive to drastically increase facility costs.
A dry etching technique employed for fabricating VLSI devices is also useful to prepare a fine trench. The dry etching technique applies a mask over a workpiece, patterns the mask by photolithography, and etches openings of the mask by active species such as ions and radicals. Reaction products adhere to the side face of each anisotropically etched trench and form a protective film, which helps the trench have a high aspect ratio.
Since this technique photolithographically patterns a mask laid over a workpiece, the workpiece must have a two-dimensional shape. Namely, the VLSI dry etching technique is not applicable for machining a three-dimensional workpiece. To fabricate as many devices as possible, it is necessary for the VLSI dry etching technique to prepare a large uniform reaction zone. Even when forming a very small trench, the VLSI dry etching technique exposes the whole workpiece to the reaction zone where plasma exists. Accordingly, parts already processed on the workpiece are always exposed to the reaction zone. This situation must be carefully taken into consideration in the course of the etching.
Namely, processing parameters must be precisely set before forming a fine trench, or else the trench will be tapered from a wide surface entrance toward a narrow bottom or will have a beer barrel shape. The processing parameters that may provide a deep trench having a required sectional shape are very difficult to find and accurately set. In practice, there is, therefore, a limit to the depth of a trench.
Reactive species produced in the reaction zone reach the bottom of the trench by diffusion and react with and etch the bottom. As the depth of the trench deepens, the reactive species hardly reach the bottom to delay an etching speed. This also limits the depth of a trench to be formed.
There are other trench machining techniques such as a laser radiation technique and an ion radiation technique, which physically bore a trench in a workpiece by melting, sublimation, or spattering. These techniques employ a beam of high energy which may not only damage the surface of a workpiece but also differ in the sizes of the entrance and bottom of a trench formed.